Miui 8 Rom's - Very Very Slow To Charge Battery

As per the title, with recent weekly ROMS, and I think probably most if not all of the MIUI 8 ones, I don't seem to be able to fast charge anymore. In fact, charging is very very slow. Not so much a problem if I switch the phone off, but certainly when on, it's very indifferent.

Any ideas what the cause of this is, and more importantly, is there a fix?